The present invention relates to a control valve used for a displacement variable compressor that is capable of changing its displacement based on a control pressure, which acts on a displacement variation mechanism.
A cooling circuit of a vehicle air conditioner generally includes a condenser, an expansion valve, which is used as a pressure reducing device, an evaporator and a compressor. The compressor draws refrigerant gas from the evaporator, compresses it and discharges the compressed gas to the condenser. The evaporator receives heat from the passenger compartment air and heats the refrigerant gas that flows in the cooling circuit. In accordance with the magnitude of the heat load and the cooling load, the heat of air that passes through the evaporator is transferred to the refrigerant that flows within the evaporator. Thus, the refrigerant gas pressure at the outlet or the downstream side of the evaporator reflects the magnitude of the air conditioning load.
A variable displacement swash plate type compressor, which is typically used in vehicles, includes a displacement control mechanism for controlling the outlet pressure of the evaporator (referred to as the suction pressure Ps) to maintain a desired target value (referred to as the set suction pressure). The displacement control mechanism performs feed back-control of the discharge displacement, that is, the angle of the swash plate, using the suction pressure Ps as the control index to achieve a flow rate of the refrigerant that corresponds to the magnitude of the cooling load. A typical example of such a displacement control mechanism is called an internal control valve. By sensing the suction pressure Ps with a pressure sensing member such as bellows, a diaphragm or the like in the internal control valve and using the motion of the pressure sensing member for positioning a valve body, the pressure (crank pressure Pc) in the swash plate chamber (also called the crank chamber) is controlled to determine the swash plate angle.
Further, since a simple internal control valve, which can have only a single preset suction pressure, cannot address fine air conditioning control needs, there are control valves that can change the preset suction pressure by external electrical control. Such control valves effect the change of the preset suction pressure by employing an actuator, such as an electromagnetic solenoid or the like, to apply force to the valve body.
A compressor to be used in a vehicle is generally driven by the vehicle engine. The compressor generally consumes the most engine power (or torque) of the several auxiliary machines that are driven by the engine. Thus, there is no doubt that the compressor is a large load on the engine. Accordingly, a typical vehicle air conditioner has a program for reducing the engine load by minimizing the discharge displacement of the compressor when engine power is needed for other purposes, such as accelerating the vehicle or driving the vehicle uphill. In an air conditioner using the variable displacement compressor including the above-described suction pressure varying valve, substantial displacement reduction is realized by changing the preset suction pressure of the control valve to a value higher than a usual preset suction pressure.
The operation of the variable displacement compressor with a preset suction pressure variable valve was analyzed in detail. As a result, it has been found that, as long as a suction pressure Ps-indexed feedback control is involved, the expected displacement reduction (that is, a decrease in the engine load) will not be necessarily realized. The graph of FIG. 14 conceptionally shows the relationship between the suction pressure Ps and the discharge displacement Vc of the compressor. As can be seen from this graph, the curve (characteristic line) between the suction pressure Ps and the discharge displacement Vc is not one kind. There are a plurality of curves in accordance with the magnitude of the heat load in the evaporator. Thus, even if a certain pressure Ps1 is given as the preset suction pressure Pset, which is a target value of the feedback control, a constant variation (xcex94Vc in the graph) is generated by the conditions of the heat load on the actual discharge displacement Vc that results from the operation of the control valve. For example, when the heat load in the evaporator is very high, even if the preset suction pressure Pset is increased sufficiently, the actual discharge displacement Vc may not be decreased enough to sufficiently reduce the engine load.
Further, as long as the above-described displacement limiting control is temporary, it is necessary to return the discharge displacement Vc of the compressor to the discharge displacement Vc that existed before the displacement limiting procedure. When the return of the displacement occurs very rapidly, an uncomfortable shock or noise is experienced by the vehicle passengers. Accordingly, it is preferred that the discharge displacement Vc be returned to normal gradually.
The graph of FIG. 15 shows various patterns of the displacement Vc of the compressor, which correlates with the load torque, over time before and after the displacement limiting control procedure. The patterns shown by the solid lines in this graph are substantially ideal linear return processes. On the contrary, as long as the control procedure is based on the suction pressure Ps, gentle linear return patterns as shown in FIG. 15 by the solid lines cannot be realized by monotonously controlling (that is, a monotonous return to the previous amount of energization of the electromagnetic solenoid) the preset suction pressure Pset. Thus, the displacement Vc abruptly increases along one of two return patterns as shown by broken lines in FIG. 15.
One pattern is a pattern in which the discharge displacement Vc immediately rises, and the other pattern is a pattern in which the discharge displacement Vc immediately rises after a considerable delay. These patterns are phenomena that are derived from the fact that the suction pressure Ps and the discharge displacement Vc of the compressor have no definite relationship. Thus, in trying to achieve a more ideal pattern for the displacement return after reducing the displacement, there was a limit based on the conventional suction pressure Ps control.
The technique of controlling the discharge displacement Vc of the displacement variable compressor based on the suction pressure Ps, which reflects the heat load in the evaporator, was an appropriate technique in attaining the original purpose of stabilizing and maintaining the compartment temperature. However, to achieve a rapid reduction in the discharge displacement and then to return to the original discharge displacement Vc in a pattern that avoids shock or noise, control must be based on something other than the suction pressure Ps.
An object of the present invention is to provide a control valve for a displacement variable compressor that is capable of controlling the discharge displacement of a compressor for stabilizing and maintaining the compartment temperature, of rapidly changing the discharge displacement and returning the displacement to normal. Specifically, the object of the present invention is to provide a control valve that accurately controls the displacement in the vicinity of the lowest discharge displacement and that permits direct control of the discharge displacement over a wide range.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a control valve for a cooling apparatus is provided. The apparatus has a compressor, which includes a displacement mechanism, an external refrigerant circuit, which is connected to the compressor to form, together with the compressor, a cooling circuit. The control valve changes the discharge displacement of the compressor by controlling a control pressure that acts on the displacement variable mechanism. The valve includes a housing, an internal passage provided in the housing, a movable valve body provided in the valve chamber for controlling the opening degree of the internal passage, a first pressure sensing structure and a second pressure sensing structure. The internal passage includes a valve chamber. The first pressure sensing structure senses the difference between two pressure monitoring points located in the cooling circuit. The difference is a primary pressure. The first pressure sensing structure transmits a force corresponding to the primary pressure to the valve body. The second pressure sensing structure senses a secondary pressure that is different from the primary pressure and applies a force corresponding to the secondary pressure to the valve body. The valve body is positioned in the valve chamber by a combination of forces corresponding to the primary pressure and the secondary pressure to control the opening degree of the internal passage.
The control valve is a valve mechanism for controlling the control pressure that is used for the discharge displacement control of the displacement variable compressor by controlling the opening degree of the passage in the valve. In the control valve of the present invention, the primary and secondary pressures are used to influence the position of the valve body in the valve chamber. The primary pressure is the differential pressure between two pressure monitoring points in the refrigerant circulating circuit. The differential pressure reflects the flow rate of the refrigerant in the circuit, that is, a discharge amount of the refrigerant from the compressor, and is used as an index for estimating the discharge displacement of the compressor. Therefore, by using the first pressure sensing structure, which presses the valve body in a specific direction based on the primary pressure (the differential pressure between two points), the primary pressure can be used as the driving force for controlling the opening degree of the valve in feedback-controlling the discharge displacement of the compressor. Accordingly, the discharge displacement, which correlates with the load torque of the compressor, can be directly controlled, and defects in the conventional, suction pressure sensing type control valve are overcome. However, if the displacement control of the compressor can be successfully achieved using only the primary pressure, there is no problem. However, there is a difficulty. In the actual refrigerant circulating circuit, there is no necessarily proportional relationship between the differential pressure between the two pressure monitoring points and the actual refrigerant flow rate. The relationship generally has a non-linear relationship (see FIG. 5) and particularly, the change of the differential pressure with respect to the change of the flow rate is extremely small in a small flow rate region. Thus, even if the positioning of the valve body is based only on the primary pressure in a case where a smaller discharge displacement of the compressor is needed, precise and stable control is difficult. Therefore, in the control valve of the present invention, the second pressure sensing structure as well as the first pressure sensing structure are used, and the valve body can be moved by the secondary pressure, which is different from the primary pressure, and the drawbacks of using only the primary pressure are mitigated.
According to the present invention, by using both the first and second pressure sensing structures, the valve body can be positioned in the valve chamber based on the combination of the primary and secondary pressures. More specifically, when the refrigerant flow rate in the refrigerant circulating circuit is small and the primary pressure is also small, the secondary pressure has a relatively stronger influence on the positioning of the valve body. On the other hand, when the refrigerant flow rate in the refrigerant circulating circuit is comparatively larger, the primary pressure has a relatively stronger influence on the positioning of the valve body. In any case, a combination force of the primary and secondary pressures act on the valve body for controlling the opening degree of the valve without being influenced by the refrigerant flow rate in the refrigerant circulating circuit. Therefore, the controllability of the opening degree of the valve is improved over substantially the whole range of the refrigerant flow rate, and direct control of the discharge displacement of the compressor over a wide range is achieved. If such a control valve is used, the displacement control of the compressor for stabilizing and maintaining the passenger compartment temperature is possible under normal conditions, and rapid change of the displacement of the compressor and the subsequent return can be achieved under exceptional conditions.